Its prepared by oxidation of benzyl alcohol by KMnO4 in presence of Na2CO3.
These three reagents are taken in a flask with 100 ml of distilled water..This mixture is boiled for about half and hour...Completion of reaction is indicated by discoloration of parmangnate...(MnO4- is reduced to MnO2 in this case).To get the acid this mixture is acidified with HCl (conc.) and 20%Na2SO3 is added with continuous shaking until all the MnO2 is completely dissolved and only the white ppt. of acid remains in the mixture which can b easily filtered out.
3 C6H5CH2OH + 4 KMnO4 = 3 C6H5COOH + 4 MnO2 + H2O + 4 KOH